Method of etching semiconductor material

ABSTRACT

The disclosure relates to a method of etching semiconductor material wherein the material is secured in an oxide coated aluminum foil which acts as an etchant mask. The portion of the material extending from one side of the foil can then be etched with a semiconductor material etchant with the remainder of the material being masked from the etchant by the foil.

BACKGROUND OF THE INVENTION

1. Field of the Invention

This invention relates to a method of making solar cells from siliconspheres disposed in a metal foil matrix wherein the cells generateelectricity upon exposure to light.

2. Description of the Prior Art

Systems for producing energy by conversion of the rays of the sun toother forms of useful energy are well known and such devices areconstantly being developed and improved due to the economics of the sunbeing the primary source of energy involved. One such system isdisclosed in U.S. Pat. No. 4,021,323 of Kilby et al. wherein a solararray composed of a transparent matrix such as glass or plastic isprovided with particles of silicon of P-type with an N-type skin on oneside thereof or N-type with a P-type skin on one side thereof embeddedin the matrix. Preferably about half of the particles are P-type withN-type skin and the remainder are N-type with a P-type skin though thisarrangement can be altered. On the backside of the matrix, the siliconparticles protruding therethrough are interconnected by appropriateelectrically conductive metallization. The silicon particles have theskin portion thereof extending through the frontside of the matrix.These arrays are immersed in an electrolyte, preferably hydrobromic acid(HBr), that contacts the frontside of the matrix. Due to the potentialdifference between the silicon particles of different conductivity typecontacting the electrolyte, a potential difference therebetween is setup under sunlight which electrolyzes the HBr into hydrogen gas whichbubbles off and bromine which remains in solution. The hydrogen gas iscollected and is a source of energy, for example, in fuel cells and thelike as is well known.

In solar arrays of this type, the silicon particles participate in theelectrolysis independently. As a result, the rate at which reactionproducts are generated by an array will not be significantly affected ifthe P-N junctions in a few particles are shorted or shunted.

Another system for producing useful energy from the sun's rays uses anarray similar to the kind described above but configured so as togenerate electricity rather than perform electrolysis. One such systemis disclosed in U.S. Pat. No. 2,904,613. Although alternate arrangementsare possible, a useful embodiment comprises a transparent matrix such asglass or plastic provided with particles of N-type silicon with a P-typeskin. The N-type cores of the particles protrude through the backside ofthe matrix and are interconnected by appropriate electrically conductivemetallization. The P-type skins protrude through the frontside of thematrix and are interconnected with an electrically conductive lighttransmissive material such as tin oxide on a fine metal gridwork. Undersunlight, a potential difference is set up between the backside andfrontside interconnections of such an array which can be suitablyconnected so as to power an external electrical load directly.

An improvement over this prior art is set forth in the application ofKent R. Carson, Ser. No. 562,782, filed Dec. 15, 1983 (TI-9744), whereinrefinements and improvements to the above noted inventions were made.However, in the present state of the art, the cost of producing solararrays in accordance with the above described prior art is relativelyuneconomical and this prior art approach has not shown a great measureof economic success to date. It is therefore imperative, in order toprovide economically viable solar arrays, that such arrays be capable ofrelatively inexpensive fabrication.

SUMMARY OF THE INVENTION

In accordance with the present invention, there is provided a method ofproducing solar arrays wherein the above noted problems in the prior artare materially reduced and wherein a solar array can be producedrelatively economically as compared with the cited prior art.

Briefly, in accordance with the present invention, a solar array isformed by providing a first sheet of flexible aluminum foil of standardtype which would likely have a native aluminum oxide on the surfacethereof. The foil is embossed in those locations wherein silicon spheresare to be positioned to form a metal matrix. The foil is then cleaned oforganics and etched to remove those thin areas where embossments werelocated to create apertures therein and provide locations thereby forinsertion of the silicon spheres. An additional etching step is used toprovide a matte surface on the foil. The foil forms the housing for thespheres which are to be applied thereto as well as the front contacttherefor. The silicon spheres, having an N-type skin over the P-type aredeposited over the backside of the foil and a vacuum chuck is providedon the frontside of the foil to suck the spheres against and partiallyinto the apertures in the foil previously formed to cut off the passageof air through the apertures. Since an excess number of spheres isinitially utilized relative to the number of apertures, all of theapertures will eventually be filled with spheres and the unused spheresare then removed by brushing or the like of the back surface of thefoil.

The silicon spheres are then bonded to the aluminum foil by use of animpact press which drives the spheres into the apertures with theequators of the spheres being positioned forward of the foil and on thefront side (side toward the sun or light) of the foil. This forcing ofthe spheres into the apertures under high force causes a tearing of thealuminum at the surfaces contacted by the silicon spheres and exposesfresh aluminum thereat. The shear caused by the movement of the siliconspheres relative to the aluminum also scrapes off the surface aluminumoxide to cause such exposed fresh aluminum. This action also removessubstantially all of the silicon oxide from the portion of the spherewhich contacts the aluminum foil and, in particular, the exposedaluminum. This action takes place with the aluminum at a temperature inthe range of about 500° C. to less than 577° C. at which time thealuminum is solid but easily deformable whereas the silicon is still arigid body at this temperature. (Temperatures above 577° C. are possibleif the impact is of short enough duration). The fresh aluminum attacksthe silicon dioxide and substantially removes it at the impact locationsduring impact. In this way, a bond between the silicon and the aluminumis provided to form an aluminum contact to the silicon N-type skinlayer. The array of foil and spheres is then cooled to ambienttemperature to allow the foil to reharden.

The backside of the foil with exposed spheres is then etched to removethe N-type skin thereat since the aluminum foil acts as a mask for thesilicon etchant, the foil itself not being very reactive due to the verythin native oxide coating normally formed thereon. The array is thenanodized by placing it in a sulfuric acid bath (about 10% H₂ SO₄) forabout 1/2 minute to provide an oxide coating on the aluminum. Next,another anodizing bath is used, the bath containing 1/2 of 1% H₃ PO₄ toseal the aluminum and anodize the silicon. There is about 10 μm of Al₂O₃ and 0.1 μm of SiO₂ grown in this manner. The back surface of thesphere is then lapped to provide a surface for making contact thereto.The lapping process roughens the surface so that a good ohmic contactwill form. A thin aluminum second foil is then applied to the lappedsurfaces and, after preheating to a temperature in the range of 500° C.to less than 577° C., the foil is impact pressed against the lappedregions and forms a contact therewith.

In the event the arrays are to be formed in a reel to reel embodiment,skins are placed between the two foils in a location between adjacentarrays prior to bonding the second foil to the spheres. In thisembodiment, the upper and lower foil are forced against but not bondedto the skin while bonding of the second foil to spheres. The foils arethen appropriately scribed over the skins on both sides of the array toprovide a foil extension portion for each foil on opposite sides of thearray. The foil extensions can then be connected together in seriescircuit relation to form an enlarged circuit.

The arrays with skins therein as above can also be scribed, separatedfrom each other and chamfered so that only one side of the rectangulararray has an outwardly extending second foil portion in the form of acontact. These contacts are connected to the first foil portions ofother arrays in any geometric configuration to provide a module withinput and output.

The result is a solar array with the predominant portion of each of thesilicon spheres disposed on the frontside of the array to provide anincreased amount of surface available for receipt of rays of the sun.Furthermore, as is apparent, the array is flexible, has a lightreflector in the aluminum foil and has been provided by utilizing arelatively small number of inexpensive materials and processing steps.

DESCRIPTION OF THE DRAWINGS

FIG. 1 is a schematic diagram of the processing steps utilized informing a solar array in accordance with the present invention;

FIG. 2 is a process schematic diagram of the process of FIG. 1;

FIG. 3 is a schematic diagram of an array interconnect procedure in aone dimensional representation;

FIG. 4 is a schematic diagram of an array interconnect procedure in atwo dimensional representation;

FIG. 5 is a schematic diagram of an array interconnect procedure in athree dimensional representation; and

FIG. 6 is a diagram of a module in accordance with the presentinvention.

DESCRIPTION OF THE PREFERRED EMBODIMENT

Referring now to FIGS. 1 and 2, there is shown a schematic diagram ofthe processing steps utilizing the features of the present invention forforming the solar array in accordance with the present invention.Initially, an aluminum foil 1 of about 2 mil thickness is provided whichis flexible and which would normally include a very thin native oxidelayer on its surface from normal exposure to the environment. While thedescription herein will be with respect to a single solar array member,it should be understood that a multiplicity of array members is providedin the total array as is exemplified by the prior art noted hereinabove.

The aluminum foil 1 is initially embossed as shown in (a) in a periodichexagonal arrangement, for example, 16 mil centers with the reducedthickness embossment 3 being of slightly smaller diameter than thediameter of the spheres to be disposed therein. The embossments can becircular or of other geometrical shapes, such as hexagonal. In the caseof a polygonal shape of the embossment, a line across the polygonthrough its center will be less than the diameters of the spheres to beapplied thereto. The foil is then cleaned to remove organics and thenetched as shown in (b) with heated sodium or potassium hydroxide toremove the region of the foil where the embossment 3 was made and toprovide an aperture 5 in its place. The embossed region 3 is removedprior to the remainder of the foil during etching because it is thinnerthan the remainder of the foil and also etches faster because it hasbeen cold worked due to the embossment that has taken place therein.This is called an aluminum matrix.

At this point the foil can optionally be textured by etching with a 50%solution of 39A etchant which is 25% HF, 60% HNO₃ and 15% glacial aceticacid to provide a matrix surface that minimizes back reflections.

A plurality of spheres of silicon 7 as shown in (c) having an N-typeskin 9 and a P-type interior 11 are deposited over the backside 13 ofthe matrix on the foil 1 and a vacuum is provided at the front side 15of the foil with a vacuum chuck to draw the spheres 7 into the apertures5. Since an excess of spheres 7 relative to the number of holes 5 isinitially utilized on the foil backside, all of the holes will be filledwith a sphere 7 and the excess spheres 7 are then removed from thebackside of the foil 1 by brushing or the like. The spheres utilizedtherein are preferably 14.5 mils in diameter and the apertures 5 asstated above, have a cross sectional diameter of less than 14.5 mils toprovide a vacuum with the foil at the foil frontside for reasons to bemade clear hereinbelow.

The spheres 7 are then bonded to the aluminum foil 1 within theapertures 5 as shown in (d) heating the foil and then use of an impactpress wherein the spheres 7 are forced quickly into the apertures 5 andcause a shearing action within the apertures which scrapes off aluminumoxide at the interior surfaces of the foil at the apertures and exposesfresh elemental aluminum. As stated, the aluminum has been heated to atemperature of about 530° C. at the time the spheres 7 are forced intothe apertures 5 so that the aluminum is reactive and somewhat viscous inmechanical properties and easily deformed. The elemental aluminumtherefore reacts with the very thin native silicon oxide layer on thespheres and removes it so that the aluminum in the foil 1 is now able tobond directly to the elemental silicon in the N-type layer 9 of thesphere to form a contact thereto.

The sphere 7 is disposed in the aperture 5 so that the equator thereofis forward of the aluminum foil 1 or on the frontside 15 thereof. Thisarrangement is made possible by the use of pressure pads which aredisposed above and below the aluminum foil 1, the pressure pads beingformed of aluminum foil about 8 mils thick coated with a release agent,such as boron nitride powder, which acts as a cushion so that the hammerof the impact press does not injure the spheres during impact. Inaddition, the pressure pads absorb the shock of the hammer. The toppressure pad, on the side 13 of the foil 1, is thicker than the bottompressure pad on the side 15 of the foil 1 to provide the offset of thesphere equator from the foil 1 as stated hereinabove. An impact energyof about 48 foot-pounds for a 2 centimeter square array has been foundto operate successfully. Accordingly, the aluminum is now bondeddirectly to the silicon as stated above.

The rear surface 13 of the foil 1 and the portion of the sphere 7 onthat side is then etched using 39A etchant as shown in (e) to remove theportion of the N-type layer 9 on the back surface of the array andexpose the P-type region. The aluminum foil 1 with native oxide thereonacts as a mask to the etchant and only permits the portion of the layer9 to the rear side of 13 of the array to be removed. The array is thenrinsed with deionized water to remove etchant and the array is thenanodized as is shown in (f) to passivate the exposed silicon and foil ina 10% H₂ SO₄ solution for about 1/2 minute at about 20 volts. The arrayis then anodized in a 0.5% H₃ PO₄ solution for about 1/2 minute at about20 volts. The time required for anodization is a function of when thecurrent in the bath goes to zero and shuts off, this having been foundto be about 1/2 minute. The use of the phosphoric acid is essential andhas been found to close holes in the aluminum oxide and provide an oxidelayer 21 of about 1,000 angstroms on the silicon surface which waspreviously etched.

The spheres 7 of the anodized array are then lapped by mechanicalabrading in well known manner on the backside 21 formed duringanodization. This lapping removes both the silicon dioxide 21 and somesilicon to level the back surface 17 of the sphere 7 and provide a roughsurface at 17 so that ohmic contacts can be formed thereon. A thin foil19 of aluminum of about 1/2 mil is then positioned over the back surface17 of each of the spheres 7 as shown in (h) so that it lies over thelapped flat regions 17, the aluminum being heated to a temperature ofabout 530° C., preferably, and in the range from about 500°-577° C. withthe proviso noted above. The heated foil 19 is then pressed against thespheres 7 by means of an impact press and a bond between the aluminumtherein which becomes exposed due to the impact and the silicon that hasbeen exposed on the back surface of the spheres 7 due to the lapping andthe impact with elemental aluminum is formed. A contact of foil 19 tosilicon region 11 is formed by bonding in the same manner as describedabove with reference to (d). Due to the anodization of the aluminum foil1, the surfaces of said foil have a thick aluminum oxide thereon tothereby prevent any short circuiting between the foil 1 and the foil 19.(A standard antireflection coating can be applied over the front surfaceof the array as shown in (i), to improve the optical absorption of thesilicon). Accordingly, it can be seen that there has been provided asolar array wherein a major portion of the silicon sphere is exposed tothe incoming rays of the sun, wherein the array is flexible and whereinthe processing utilized and the materials utilized are relativelyinexpensive and few in number.

In actual processing procedures, the array as disclosed hereinabove cannormally be provided in a reel-to-reel embodiment rather than asseparate arrays. The arrays will then be formed into modules which maybe, for example, one meter by two meters in size and then tested in suchdesign. Each array formed in the manner noted hereinabove would normallybe on the order of 10 centimeters on each side.

To provide the solar array described hereinabove in reel-to-reel formand then form modules therefrom, a procedure will be followed as setforth in FIGS. 3 through 6. Referring first to FIG. 3, there is shown aone dimensional representation of an array interconnect system wherein,in FIG. 3(a), there is shown a single array 30 with spheres 31 securedin the front contact foil member 33 and with the back foil member 35 notyet attached to the spheres. Shims 37 are inserted between arrays 30 asmore clearly shown in FIG. 4(a). As can be seen from FIG. 4(a), thefront foil 33 will be of lesser dimension than the back foil 35 forreasons that will become apparent hereinbelow.

Referring now to FIG. 3(b), it can be seen that the back foil 35 is nowin contact with the spheres 31 as well as with the shims 37, the topfoil 33 also being in contact with the shims. This is accomplishedduring the step (h) of FIG. 1 wherein the back foil 35 is bonded to thespheres 31 as a portion of that process step. The foils 33 and 35 willnot adhere to the shims 37 and merely be in contact therewith. The foilswill then be scribed at the location of the V-shaped members of FIGS.3(b) over the shims to provide an arrangement as shown in FIG. 3(c) andin FIG. 4(b) after the arrays have been separated from each other andthe shims removed. The array as shown in FIGS. 3(c) and 4(b) is thenchamfered as is shown in FIG. 4(c) to provide four tabs which are aportion of the back foil 35, these tabs being located on each side ofthe array square and being labelled A, B, C and D. The tabs B, C, D, arethen folded under the array as shown in FIGS. 3(d) and 4(d) and thearray is then secured to a subsequent array by bonding the tab A to oneof the tabs B, C or D of a subsequent array by ultrasonic bonding or thelike as shown in FIG. 3(e).

The interconnections step can be provided as shown in the threedimensional representation arrangement of FIG. 5 wherein one of thearrays with tab A extending therefrom is positioned so that the tab Acontacts one of the tabs B, C or D of a further array with thisprocedure being continued in a straight line or other path to provide acomplete module. A completed module is shown in FIG. 6 wherein tabs Aare secured to tabs B, C or D of adjacent arrays 30 to provide a backand forth path which forms a series circuit of sixty such arrays. Alsoprovided are tabs for input 41 and output 43 to the module.

After formation of the module of FIG. 6, with reference to FIG. 2, themodule is tested and, if the test is successful, the module proceeds tobe mounted on a backing material or the like and the tabs are thenultrasonically bonded together at a bonding station, after which themodule is encapsulated to provide appropriate environmental sealing. Theencapsulated module is then again tested in standard manner whereby anoperational module is provided for use.

Though the invention has been described with respect to a specificpreferred embodiment thereof, many variations and modifications willimmediately become apparent to those skilled in the art. It is thereforethe intention that the appended claims be interpreted as broadly aspossible in view of the prior art to include all such variations andmodifications.

What is claimed is:
 1. A method of etching a semiconductor materialcomprising the steps of:(a) providing said semiconductor material; (b)masking the portion of said material not to be etched with oxide coatedaluminum; and (c) etching said material with an etchant for saidmaterial which is relatively inert to said oxide.
 2. A method as setforth in claim 1 wherein said oxide is aluminum oxide.
 3. A method asset forth in claim 1 wherein said aluminum is a foil and said materialextends outwardly on both sides of said foil.
 4. A method as set forthin claim 2 wherein said aluminum is a foil and said material extendsoutwardly on both sides of said foil.
 5. A method as set forth in claim1 wherein said material is silicon.
 6. A method as set forth in claim 2wherein said material is silicon.
 7. A method as set forth in claim 3wherein said material is silicon.
 8. A method as set forth in claim 4wherein said material is silicon.
 9. A method as set forth in claim 1wherein said step of etching is provided using an etchant composed ofHF,HNO₃ and acetic acid.
 10. A method as set forth in claim 2 whereinsaid step of etching is provided using an etchant composed of HF,HNO₃and acetic acid.
 11. A method as set forth in claim 3 wherein said stepof etching is provided using an etchant composed of HF,HN03 and aceticacid.
 12. A method as set forth in claim 4 wherein said step of etchingis provided using an etchant composed of HF,HNO₃ and acetic acid.
 13. Amethod as set forth in claim 5 wherein said step of etching is providedusing an etchant composed of HF,HNO₃ and acetic acid.
 14. A method asset forth in claim 6 wherein said step of etching is provided using anetchant composed of HF,HNO₃ and acetic acid.
 15. A method as set forthin claim 7 wherein said step of etching is provided using an etchantcomposed of HF,HNO₃ and acetic acid.
 16. A method as set forth in claim8 wherein said step of etching is provided using an etchant composed ofHF,HNO₃ and acetic acid.